This invention relates to breathable films, or films capable of transmitting water vapor but generally resistant to the passage of water in a liquid form.
Breathable films have been known and used for many years in the production of, for example, water proof clothing, to allow escape of water vapor from the surface of the skin of the wearer outwardly. Vapor breathable films can be broadly classified as being a microporous film or a monolithic film.
A microporous film has a large number of pores formed in the film using special processing conditions. One method of obtaining microporsity is described in U.S. Pat. No. 3,870,593. A quantity of powdered inert material such as calcium carbonate is incorporated into the film prior to extrusion. After extrusion, the film is drawn, causing small pores to be established at the sites of the filler particles. As described in U.S. Pat. No. 4,308,303 after production of the microporous film, the film may be coated with fibers to produce a composite. The lamination of microporous films to fabrics is also well known.
Microporous films are not suitable for certain end uses and are not desirable for others due to their nature of indiscriminately passing all gases, vapors and other agents such as pathogens. These films, for example, are generally not suitable as viral barriers, and garments and other articles having a viral barrier yet good breathability are highly desirable in the medical and other industries where exposure to blood is commonplace.
A second class of breathable films can be referred to as monolithic films, in which the film is continuous and free of pores. Monolithic breathable films are capable of allowing the transfer of certain gases and liquid vapors due to chemical absorption, transfer through the film thickness and release on the opposite surface. For films having a high rate of moisture transmission, the rate of transfer is very rapid, and is driven by the relatively high concentration and pressure of vapor on one side of the film. This mechanism of transfer is described in U.S. Pat. No. 5,445,874, which discloses thin films of certain polyurethanes which possess moisture vapor transmission rates (MVTR) higher than the human skin, allowing the film to be used as an outer layer in burn dressings.
Several thermoplastic resins are currently available which allow the extrusion of films having a high MVTR. Examples include polyurethanes, copolyesters and polyester elastomers. These resins can be extrusion coated on a porous support substrate such as a woven or nonwoven fabric which is used to make protective clothing and other articles.
Apart from being relatively expensive in comparison to other film forming thermoplastic polymers, the above noted polymers are not suitable for certain types of end uses. The only way to change the MVTR of the film is to increase or decrease the thickness of the film, with thinner films providing higher MVTR's. Some films cannot be laminated directly to certain fabrics, for example, by extrusion coating, or the bond between the fabric and the film is unduly weakened by use. Also, these breathable films tend to be harsh and noisy when combined with fabrics and may not have an attractive visual appearance. Since all these properties are important from the viewpoint of manufacture and successful end use, additional improvements in the field of breathable monolithic films are needed.